Despite all the evidence available, all the time that has passed and all the expert opinion offered on this forum, there has been no solution to this mystery. We know what happened, but we do not know why. So we speculate.
Having ruled out almost everything else, it looks like the pilots broke all the rules and tried to fly the airplane so far outside its operating limits and their own, they crashed it. In the worst case, they were having a little fun, playing a dangerous and illegal game that got out of hand. Hard to believe, but if there is a more forgiving explanation on the cockpit voice recorder we should be hearing it sooner rather than later.
If not, and this awful conclusion turns out to be correct, this crash was not an accident. It was a systemic failure. We don’t speculate on that. Or at least I don't